This invention relates to an arc-shorting device which is particularly, although not exclusively, applicable to fluid-filled metal-clad switchgear employing SF.sub.6 gas or other gas or liquid as the electrical insulating medium.
Should an internal arc occur in an enclosure of a metal-clad switchgear installation, a very rapid increase in the pressure of the insulating fluid would occur and the burning arc would cause damage to the installation. The pressure rise would depend on factors such as the magnitude of the fault-current and the arc-voltage, the duration of the arc and the volume of the enclosure. Such an occurrence could cause an explosive failure or a burn-through of the enclosure, which could release a quantity of high-pressure, ionised and perhaps toxic gas.
This eventuality must be guarded against by limiting the pressure rise, removing the arc as quickly as possible or else taking steps to minimise the effects of the arc. This may be done by means of a protection system which detects the fault and opens an associated circuit-breaker to interrupt the current and remove the arc.
It is also known to use a remotely-located earth-switch, which effectively short-circuits the arc.
Even with the fastest protection systems, it may take up to 80 milliseconds to clear the fault arc. Not all systems could respond as quickly, and if a fault occurred in the protective system or in the circuit-breaker itself the operation of a back-up protection system could result in an arc duration of 500 milliseconds or more.
The operation of high-speed earth-switches via associated protective circuitry can also lead to excessive arc durations.
A large gas volume in the enclosure is desirable to limit the pressure rise but is not always practicable from a service and economic point of view. A further solution involves connecting small enclosures to adjacent larger enclosures by employing collapsible insulating barriers, which are arranged to fail when a certain pressure is reached. This system entails careful co-ordination of the mechanical strengths of components and spreads the effects of arcing.
Other proposed methods of overcoming this problem have included the provision of bursting discs or diaphragms. These must be large enough to provide the necessary pressure relief, but since they are arranged to vent the enclosure externally, they defeat a major objective, which is to minimise any external manifestation of the arc.